Beijing: A sudden security clampdown at the main international airport here that led to intensified checks caused as many as 1000 people to miss their flights or left them waiting for hours.

Long queues of passengers were seen waiting at the security check points at the Capital International Airport on Sunday as the sudden increase in security meant the staff took far longer time to check passengers even for the domestic flights.

The security was also stepped up at the Shanghai airport. At least 1,000 passengers missed their flights or were kept waiting for hours at the airport, state-run agency reported. The exact number of people or flights affected could not be verified, it said.

The sudden increase of security followed speculation that agencies had foiled a "terrorist attack attempt" last month at Urumqi airport in the Xinjiang Uyghur Autonomous Region after uncovering a plot to smuggle daggers hidden inside walking sticks, it said.

Gao, a university professor told the agency that he missed his flight to Hohhot in Inner Mongolia Autonomous Region because many passengers were not ready for the tightened security.

"They even asked me to remove my pants," Gao said, adding that he had not been informed of the security upgrade.Traffic at the airport appeared to have returned to normal by 1 pm local time yesterday.

"There is no need to put the airport on special notice since it is functioning normally," an employee at the BCIA told the daily.

A similar order for heightened security was also received in Shanghai last morning, Tian Lu, a press officer with the Shanghai Airport Authority, told the agency.

The Shanghai-based post warned that passengers at the city's Pudong and Hongqiao airports would have to remove belts and shoes because "airports across the nation have raised their security level today".

The airport of Urumqi has enforced tightened security checks since early August, ahead of the China-Eurasia Expo that runs from Thursday to September 5.

Passengers must go through two rounds of checks: upon entering the terminals and then at the normal security check, where they are asked to remove their shoes and belts.